A critical measurement in the operation of nuclear reactors is that of in-core flux density. Neutron detectors responsive to neutron flux changes are well known and broadly classified into two categories as "prompt-responding" and "delayed-response" types. The prompt detector instantaneously respond to neutron flux changes while the detector signal of the delayed detector reaches equilibrium at a significantly measureable time following termination of the neutron flux change. The delayed-response detector, which are more accurate than the prompt responding types, are used to provide a history of power distributions and variations during power operation modes but do not provide a fast enough signal to be used for safety functions. The less accurate prompt-responding type detectors, on the other hand, must be intermittently or continuously calibrated to assure an accurate signal. Generally, therefore, the prompt detectors have been used outside of the reactor to assure accessibility for calibration. This, however, inherently leads to a less accurate determination of the incore flux.
Alternately, in the prior art, where incore prompt detectors are used, such detectors are calibrated through the use of a moveable calibration detector system. The calibration detectors remain outside the core until called upon for calibration of the fixed incore detectors. The calibration detectors are then inserted by mechanical drive units into the reactor core, by way of calibration tubes of the fixed detector assemblies, and the calibration of the fixed incore detectors is accomplished. However, calibration of fixed detectors using a moveable calibration system has occurred at infrequent intervals because of wear-and-tear on the moveable system.
A further disadvantage of prior art detectors is that less power from a given amount of nuclear reactor fuel is obtained. An increase to optimum power density in a reactor core can be achieved only if the safety system can provide instantaneous protection by responding promptly to power changes in each fuel channel. Hence, it is important that such incore detectors as are used in the sensing of core power density be capable of prompt response to changes in power density as manifested by changes in local neutron fluxes. The output signals from such detectors must necessarily represent incore flux conditions that are instantaneously current rather than flux conditions that actually existed several seconds or more in the past.